Hidden hinge MEMS device

ABSTRACT

The present invention relates to a method for manufacturing a MEMS device, including the actions of: providing a substrate having a back and front surface essentially in parallel with each other, defining in said substrate at least one hidden support by removing material from said substrate, connecting said at least one hidden support onto a wafer with at least one actuation electrode capable to actuate at least a part of said substrate, wherein a rotational axis of said reflective surface is essentially perpendicular to said hidden support. The invention also relates to the MEMS as such.

TECHNICAL FIELD

The present invention relates in general to techniques for forming anintegrated device, e.g. a semiconductor device, and in particular to amethod for manufacturing micro mirrors with a hidden hinge and to an SLMcomprising such micro mirrors.

DESCRIPTION OF THE BACKGROUND ART

It is well known in the current art to build spatial light modulators(SLM) of a micro mirror type U.S. Pat. No. 4,566,935, U.S. Pat. No.4,710,732, U.S. Pat. No. 4,956,619. In general two main principles forbuilding integrated devices, such as micro mirror SLM, have beenemployed.

An integrated circuit (IC) is manufactured to a finished state, and thenthe micro mirrors are manufactured on said IC. The micro mirrors arebuilt onto the IC wafers. An advantage with this approach is that socalled IC foundries can be, used, which presents a very cost efficientmanufacturing of the electronics wafers. A disadvantage is that there isa very restricted selection of materials and methods that are usable forthe manufacturing of the micro mirrors, because there is an uppertemperature limit of about 400° C., above which the electronics will bedamaged. This makes the manufacturing of micro mirror having optimalperformance more difficult.

Another way of building micro mirror SLM's is at the end of the processfor making the IC, micro mirror manufacture may be started on the samewafers. An advantage with this approach may be that there is a greaterfreedom of selecting materials, methods and temperatures for themanufacture of micro mirrors having good performance. A disadvantage isthat the IC wafers cannot be manufactured in standard IC foundries,because they have very strict demands on a process of manufacturing tobe standardized in order to be able to maintain the quality in theprocess.

Yet another way of building micro mirror SLM's may be to manufacture theIC on a first wafer and a micromirror array on a second wafer. Saidfirst and second wafers may be attached to each other by means ofbonding. One problem with such a method may be the tight demands on thealignment between said first and second wafers, a misalignment mayaffect the functionality of one or several pixels.

Micromirrors in SLM may be made of aluminum due to its good opticalperformance. However, there may be some drawbacks by using mirrors madeof Aluminum such as: the mirrors may not be perfectly flat, a mirrorheight may differ between mirrors, the mirrors may bend when tilted, themirrors may sag when tilted, the mirrors may have a built inpredeflection which may be different from mirror to mirror, the hingemay have anelastic behaviour.

Therefore, there is a need in the art for an improved method formanufacturing micro electrical/mechanical/optical integrated devices.

SUMMARY OF THE INVENTION

In view of the foregoing background, the method for manufacturingintegrated devices, such as for example micro mirror SLM's, is criticalfor the performance of such devices.

Accordingly, it is an object of the present invention to provide animproved manufacturing method and/or design for an integrated devicewhich overcomes or at least reduces the above mentioned problems.

In an example embodiment, the invention provides a method of formanufacturing a MEMS device, including providing a substrate having aback and a front surface essentially is parallel with each other,defining in said substrate at least one hidden support by removingmaterial from said substrate, connecting said at least one hiddensupport onto a wafer with at least one actuation electrode capable toactuate at least a part of said substrate, wherein a rotational axis ofsaid reflective surface is essentially perpendicular to said hiddensupport.

In another example embodiment, the invention provides a MEMS devicecomprising a substrate having at least one reflective surface, at leastone hidden support formed out of the same material as said substrate, atleast one actuation electrode provided on a wafer capable to actuatesaid reflective surface, wherein said wafer is connected to saidsubstrate and a rotational axis of said reflective surface isessentially perpendicular to said hidden support.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1-20 shows an example embodiment of process steps in consecutiveorder according to the present invention for manufacturing a micromirrorwith a hidden hinge.

FIG. 21 illustrates a 3D view of an example embodiment of a micromirroraccording to the present invention.

FIG. 22-32 illustrates another example embodiment of an inventivemanufacturing process for the inventive MEMS device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purpose of this application, the terms “wafer” and “substrate”are used interchangeably, the difference between them merely amountingto dimensions thereof.

The method according to the present invention is particularly suited forthe manufacturing of micro mirror Spatial Light Modulators. However, itwould be applicable to a wide variety of MEMS, thermal and non thermaldetector devices, such as, but not limited to, quantum well detectors,pyroelectric detectors, bolometers, etc. It is particularly suitablewhen for the same reason it is not possible to process/pattern/deposit astructure (e.g. a micro mirror array) directly on a substrate, whereanother structure (e.g. steering electronics) is present. This can e.g.be the case if the structure provided on said substrate, is temperaturesensitive to the process temperature for the processing of the structureto be provided thereon, or when the substrate is poly crystalline andthe elements that is grown on top of the substrate have to bemonochrystalline.

FIG. 1 illustrates schematically a first process step according to anexample embodiment of the invention for forming a MEMS device with ahidden hinge. A hidden hinge is a hinge that is hidden by a reflectivesurface in said MEMS device when viewing said MEMS device from above,i.e., a top view. A starting material is a wafer 130, which may be madeof single crystalline silicon or SOI. On top of said wafer 130 isprovided a layer of mask material 120, for instance silicon oxide. Saidmask material 120 may at least partly be covered with a resist film 110.In said first process step, standard photo-lithography may be used forhinge definition 140 in the mask material 120. RIE (Reactive IonEtching), which may be CF₄, may be used to remove both exposed areas ofthe resist film 110 and underlying mask material 120.

Definition of hinges in the substrate 130 may be made by using DRIE(Deep RIE), FIG. 2. Before said hinges is defined in said substrate 130the resist film 110 may be removed in a resist remover. Prior to thedefinition of the hinges in the substrate 130 said substrate may bedipped in 2% HF. The DRIE may be the well known Bosch process. The mostsimplified process consist of just an anisotropic DRIE etch followed, byan isotropic RIE to form the hinges. Prior to said definition of saidhinges in said substrate 130, a layer of silicon oxide 150 may beprovided on the opposite side of said substrate 130 with respect towhere said hinges is to be defined, alternatively said layer of siliconoxide 150 may be provided on said opposite side after the definition ofsaid hinges in said substrate 130.

In a next step, a passivation of processed surface may be performed,FIG. 3. A dry oxidization may be made in order to relax a stress in thesilicon during local oxidation, not illustrated in FIG. 3. Saidoxidation may be optional to increase accuracy and reduce surfaceroughness. A PECVD (Plasma Enhanced Chemical Vapour Deposition) ofsilicon nitride 160 may be made in order to act as an oxidation barrierin a following LOCOS (LOCal Oxidation of Silicon) step. A PECVD ofsilicon oxide 170 to act as an etch protection may be performed as aprotection in a following DRIE step.

In FIG. 4 a removal of passivisation layers (silicon nitride 160 andsilicon oxide 170) on horizontal surfaces has been performed and adefinition of length of hinges in substrate 130 has been made. Thepassivisation layers may be removed by means of RIE with highdirectionality (low pressure and high RF power). Revealed surfaces ofsubstrate 130 may be etched by means of DRIE (Bosch process) in order todefine said length of said hinges.

In a next step, illustrated in FIG. 5, a thermal oxidation has beenperformed to define a width of the hinges. LOCOS may be used totransform part of the substrate 130 in the hinges to silicon oxide 180.

In FIG. 6 a removal of passivisation layers and mask material and aplanarization of substrate have been performed. Passivisation layers160, 170, 180 and mask material 120 may be etched away in BOE (BufferedOxide Etch). Polyimide (PI) 190 may be spun on top of the substrate 130filling the cavities therein. Reduced pressure or vacuum may be used inorder to make sure that said PI will fill said cavities. Said PI may becured at an elevated temperature. Unwanted PI may be removed with O₂plasma.

A mask material 200 is deposited on top of said substrate 130, FIG. 7.Said mask material may be aluminum and the deposition may be performedby evaporation.

On top of said mask material a film of resist may be provided. Standardphotolithography may define areas 220 where the definition of the mirrorseparation trenches will be in the substrate 130, see FIG. 8. Thealuminum beneath the exposed resist may be removed with RIE (SiCL₄/Cl₂). The separation trenches may also be formed as a first step byusing a trenched (usually filled by oxide) SOI wafer as used for trenchisolation of electronics.

Unexposed resist film may be removed with acetone. On top of the maskmaterial 200 and said relieved substrate 130 a layer of silicon oxide isprovided, see FIG. 9. Said layer may be provided by means of PECVD.

On top of said silicon oxide layer 230 a layer of resist 245 isprovided. Standard lithography may define mirror separation trenches 240and electrode trenches 250 in mask material 230 (silicon oxide). Thesilicon oxide 230 may be etched by means of RIE, for instance CF₄.

In FIG. 11, mirror separation trenches 260 have been formed in thesubstrate 130. The resist 245 has been removed by using for instanceacetone. Said mirror trenches 260 in said substrate 130 may be made byusing the Bosch process.

In FIG. 12, electrode trenches 255 in the aluminum layer have beendefined. Said electrode trenches may be defined by Beans of RIE, forinstance SiCl₄/Cl₂.

PI has been introduced in said mirror trenches 260 in FIG. 13. UnwantedPI may have been removed by using O₂ plasma.

Electrode trenches 257 in substrate 130 have been made in FIG. 14. Saidelectrode trenches 257 may have been made by using the Bosch process.

In FIG. 15, silicon oxide 270 may have been PECVD to act as etchprotection in the following isotropic DRIE step.

In FIG. 16, passivation layer 270 on horizontal surfaces have beenremoved. The removal of said passivation layers may be performed byusing RIE.

A release of a foot structure has been made in FIG. 17. Isotropic RIE(may also be substituted by wet isotropic or anisotropic etching) of thesubstrate 130 has been made in order to release the foot of he mirrorwith an under etch of the material between hinges. By removing thematerial between the hinges, an applied actuation force to deflect themirror to a certain deflection state may be heavily reduced. Theisotropic etch also removes unnecessary material in the mirror, i.e.,reduces its weight, which may affect the speed of setting the mirrorfrom one state to another and its self oscillating frequency.

In FIG. 18, the passivation layers 230, 270 and mask layers 200 havebeen removed. These layers may be removed by means of BOE.

In FIG. 19, a substrate 300 with actuation electronics 310 has beenattached to said substrate 130. At least one hinge is attached to saidsubstrate 300. The substrate 300 has en elevated structure 320 forattaching said hinge(s) (alternatively the electrode areas of thesubstrate 130 may be lowered). Beside said elevated structure 320actuation electronics 310 is provided. Here one can easily see thatthere is a big attachment area for the substrate 130 to attach to saidsubstrate 300. Even if there may be a slightly mismatch between said twosubstrates, a successful attachment may nevertheless be performed. Saidattachment may be a low temperature oxygen plasma assisted bonding,adhesive bonding (gluing), soldering, eutectic bonding, fusion bonding(direct bonding), glass frit bonding, anodic bonding.

In FIG. 20 the buried oxide 280 has been removed from the substrate 130.This buried oxide may be removed by means of BOE. The mirrors 132 may bereleased by removing the PI. PI may be removed by using O₂-plasma. FromFIG. 20 one may see that the mirror structure is relatively stiff. Thisis due to the vertical part 136, which will strongly affect thestiffness and planarity of a mirror surface. The hinge 134 may bedesigned to be as stiff or weak as desired. The mirror may be made of apure single crystalline material, for instance silicon. Otheralternative material of the mirror may be polysilicon, quartz,three-five materials, SiC. In order to improve the electricalconductance, said mirror material may be doped if made of asemiconducting material. A surface facing towards the electronics insubstrate 300 may be coated with an electrically conducting material.

FIG. 22-32 illustrates an alternative example embodiment of an inventivemanufacturing process for the inventive MEMS device. In FIG. 22 astarting material is a wafer 130, which may be made of singlecrystalline silicon or SOI. On top of said wafer 130 is provided a layerof mask material 120, for instance silicon oxide. Said mask material 120may at least partly be covered with a resist film 110. In said firstprocess step, standard photo-lithography may be, used for definingtrench separation 300 in the mask material 120. RIE (Reactive IonEtching), which may be CF₄ may be used to remove both exposed areas ofthe resist film 110 and underlying mask material 120. Definition oftrench separation in the substrate 130 may be made by using DRIE (DeepRIE), FIG. 22. Before said trench separations 300 are defined in saidsubstrate 130 the resist film 110 may be removed in a resist remover.Prior to the definition of the hinges in the substrate 130 saidsubstrate may be dipped in 2% HF. The DRIE may be the well known Boschprocess. The most simplified process consist of just an anisotropic DRIEetch followed by an isotropic RIE to form said trenches. Prior to saiddefinition of said trenches in said substrate 130, a layer of siliconoxide 150 may be provided on the opposite side of said substrate 130with respect to where said trenches 300 are to be defined, alternativelysaid layer of silicon oxide 150 may be provided on said opposite sideafter the definition of said trenches in said substrate 130.

Said trenches 300 may be filled by first spinning Polyimide (PI) 310 ontop of the substrate 130 filling the cavities therein. Reduced pressureor vacuum may be used in order to make sure that said PI will fill saidcavities. Said PI may be cured at an elevated temperature. Unwanted PImay be removed with O₂ plasma, see FIG. 24.

FIG. 25-29 illustrates the process steps for defining the buried orhidden hinges. In FIG. 25, standard photo-lithography may be used fordefining entrance holes 310 in the mask material 120. RIE (Reactive IonEtching), which may be CF₄, may be used to remove both exposed areas ofthe resist film 110 and underlying mask material 120.

In FIG. 26 a dry etch may be used for defining holes 320 in thesubstrate 130. After said holes 320 have been defined in said substrate130 a stripping of said resist 110 may be performed. After stripping theresist a layer of oxide may be deposited in order to arrange a layer ofoxide in said holes 320.

In FIG. 27 a dry etch may be used to etch horizontal surfaces of saidlayer of oxide.

In FIG. 28 an isotropic dry etch may be used to create a cavity 330 andburied hinges 340 in the substrate 130. In FIG. 29 the oxide layer hasbeen removed in BOE. In FIG. 30 an alternative cross section of thestructure is illustrated, the cross section is illustrates to the leftto FIG. 30. In FIG. 31 the substrate 130 may be bonded onto a wafer 400with actuation electrodes 410. The oxide layer 150 may be removed bymeans of BOE, and the polyimide by dry etching in O₂ plasma, see FIG. 32

FIG. 21 illustrates a perspective view of an example embodiment of amirror structure 132 according to the present invention. Said mirrorstructure comprising a mirror surface 135, supports 134, cavity 131,base element 136, a first leg 142 and a second leg 144. The mirrorstructure 132 may have at least one cross section which is as thick asthe original substrate 130, which in this particular embodiment may bethe distance from the mirror surface 135 to an electrostaticallyattraction surface 145, 147. This may give the mirror structure goodmechanical properties, such as high stiffness, i.e., the mirror surfaceis essentially rigid while being in a deflected position. The supports134 may be thin pillars. The supports may support the mirror structure132 and at the same time function as a hinge. In the illustrated exampleembodiment in FIG. 21 said support is arranged so that the rotationalaxis is essentially in the middle of the structure. In an alternativeexample embodiment said rotational axis may be arranged off center,which may be achieved by displacing the supports from a center position.An axis of rotation of the mirror surface 135 may be parallel to themirror surface and perpendicular to the support 134.

The base element 136 and the support 134 may be denoted a hidden hinge.In another embodiment the base element 136 is minimized so that thesupport 134 only may be denoted the hidden hinge (hidden support). Thecross section of said pillars may be polygonal, for instance triangularor rectangular. The base element 136 may be attached to the supports134. A bottom surface of the base element 143 may be attachable toanother surface, such as a wafer with steering-electronics. The legs142, 144 may have surfaces 146, 148 essentially perpendicular to themirror surface 135. The cavity 131 may be formed by means of anisotropic etching process according to the example embodiment above. Themirror structure 132 may be doped. The doping is preferably made priorto defining the cavity 131 and supports 134, i.e., the substrate to beused for defining said mirror structure may be doped. In this embodimentthe electrostatically attraction surface 147 may be used to rotate themirror structure 132 clockwise. The electrostatically attraction surface145 may be used to rotate the mirror structure 132 counter clockwise,i.e., said structure may be rotated clockwise or anti clockwise from nonactuated state. The surface 143 of the base element 136 may be atanother level compared to the electrostatically attraction surfaces 145,147.

In the embodiments disclosed hereinabove the actuation of the mirrorelement has been electrostatic. However, other means of actuating themirror element is possible such as thermal, piezoelectric or magnetic,which is well known for a skilled person is the art.

Thus, although there has been disclosed to this point particularembodiments of the method of combining components to form an integrateddevice, it is not intended that such specific references be consideredas limitations upon the scope of this invention except in-so-far as setforth in the following claims. Furthermore, having described theinvention in connection with certain specific embodiments thereof, it isto be understood that further modifications may suggest themselves tothose skilled in the art, it is intended to cover all such modificationsas fall within the scope of the appended claims.

1.-9. (canceled)
 10. A MEMS device comprising: a first substrate havingat least one reflective surface; at least one hidden support, saidhidden support being hidden underneath said reflective surface; at leastone leg, wherein said at least one leg is having a first surfaceessentially perpendicular to said reflective surface and a secondelectrostatically attractable surface essentially in parallel with saidreflective surface, wherein said electrostatically attractable surfaceis used to rotate the reflective surface around a rotational axis; andat least one actuation electrode provided on a second substrate, saidsecond substrate being operable for actuating said reflective surface;wherein said second substrate is connected to said substrate and saidrotational axis of said reflective surface is essentially perpendicularto said hidden support.
 11. (canceled)
 12. The MEMS device according toclaim 10, wherein said hidden hinge is essentially perpendicular to saidreflective surface.
 13. The MEMS device according to claim 10, whereinsaid MEMS device is an SLM.
 14. The MEMS device according to claim 13,wherein said SLM comprises at least one mirror element with essentiallya reflective surface.
 15. The MEMS device according to claim 13, whereinsaid SLM comprises at least one mirror element with a surface havingdiffractive properties.
 16. The MEMS device according to claim 14,wherein said at least one mirror element is deflectable by bending saidhidden support while said at least one mirror element is keptessentially rigid when deflected. 17.-18. (canceled)
 19. The MEMS deviceaccording to claim 10, wherein said first substrate is doped. 20.(canceled)
 21. The MEMS device according to claim 10, wherein saidreflective surface is rotatable in a first and a second direction from anon actuated state.
 22. The MEMS device according to claim 10, whereinsaid actuation electrode is capable to actuate said reflective surfaceelectrostatically, magnetically, piezoelectrically or thermally.
 23. TheMEMS device according to claim 15, wherein said at least one mirrorelement is deflectable by bending said hidden support while said atleast one mirror element is kept essentially rigid when deflected. 24.The MEMS device according to claim 10, wherein said second substrate isa wafer.